1. Field of the Invention
The present invention generally relates to a method for manufacturing a fuel cell, and more particularly to a method for manufacturing a membrane electrode assembly of the fuel cell with solvent pretreatment to the electrolyte membrane of the membrane electrode assembly.
2. Description of the Prior Art
A fuel cell converts chemical energy into electrical energy and thermal energy by means of chemical reaction between hydrogen-containing fuel and oxygen. Benefits of the fuel cell include low pollution, high efficiency, high energy density and simple fuel recharge. Applications of the fuel cells include electrochemical engines, portable power supplies, standby power supply facilities, power generating systems, and so on.
The chemical reaction of a fuel cell requires the presence of an electrolyte, electrodes and catalysts. Based on the electrolyte, the fuel cell is classified as AFC, PAFC, MCFC, SOFC, and proton exchange membrane. During recent years, the proton exchange membrane type fuel cell is one of the most intensely-researched fuel cell. The proton exchange membrane may be classified as PEMFC and DMFC. The difference between PEMFC and DMFC is the fuel that they take. A PEMFC uses hydrogen or reformed gases containing rich hydrogen while a DMFC uses methanol solution.
A typical proton exchange membrane type fuel cell comprises a seven-layered structure, including a central electrolyte membrane for the transmission of protons, two catalyst layers on opposite sides of the electrolyte membrane in which the chemical reactions occur, two gas diffusion electrodes stacked on the catalyst layers comprising low porosity carbon paper or cloth through which reactants and reaction products diffuse in and out of the cell, and two flow field plates stacked on the gas diffusion electrodes. The flow field plates are made of carbon plates, metal plates or composite graphite fiber plates. Gas guide channels are defined on the gas diffusion electrode facing sides of the flow field plates. Reactants and reaction products are guided into/out of the cell through the flow field plates. The structure mentioned above forms a basic fuel cell unit. Conventionally, a fuel cell stack comprises a number of basic fuel cell units arranged to form a stack and is serially connected together. If desired, cooling plates and humidifying plates may be added to ensure the operation and performance of the fuel cell stack.
Examples of fuel cells and the manufacturing techniques thereof were disclosed in U.S, Pat. Nos. 5,252,410, 5,399,184, 5,523,177, 5,683,828, 5,723,173, 5,723,288, 5,869,201, and 6,010,606.
Porous material is used as the electrode of the fuel cell for the reaction gas coming in and the product gas going out, so called Gas Diffusion Electrode.
FIG. 1 shows a conventional basic fuel cell unit, comprising a central electrolyte membrane 10. One side of the electrolyte membrane 10 is coated with a cathode catalyst layer 21, and the other side of the electrolyte membrane 10 is coated with an anode catalyst layer 22. Two gas diffusion electrodes 31 and 32, which are usually made of carbon cloth or carbon paper, are formed on the outer side of the catalyst layers 21 and 22 respectively. The Conventional process for manufacturing the fuel cell unit is coating the catalyst slurry on the inner side of the gas diffusion electrodes 31 and 32 to form catalyst layers 21 and 22 respectively. Then, the electrolyte membrane 10 is interposed between the cathode gas diffusion electrode 31 and the anode gas diffusion electrode 32 coated with catalyst layer to form a basic fuel cell unit 1. It is found that the conventional method has a serious disadvantage that the catalyst slurry is easily permeating into the carbon cloth or carbon paper of the gas diffusion electrode, and the thickness of the catalyst layer coated is not easy to be controlled.
In another manufacturing method in prior art, the catalyst slurry is coated on both sides of the electrolyte membrane 10, as shown in FIG. 2. The structure fabricated by this approach is so called Membrane Electrode Assembly (MEA). In this process, the surfaces of both sides of the electrolyte membrane 10 are first coated with a cathode catalyst layer 21 and an anode catalyst layer 22 respectively. Then, the electrolyte membrane 10 is interposed between the gas diffusion electrodes 31 and 32 to form a basic fuel cell unit 1.
FIG. 3 is a left side elevational view showing the electrolyte membrane 10 and the cathode catalyst layer 21 shown in FIG. 2. FIG. 4 is a right side elevational view showing the electrolyte membrane 10 and the anode catalyst layer 22 shown in FIG. 2.
FIG. 5 is a perspective view showing a fuel cell stack comprising a basic fuel cell unit 1, an anode gas distribution plate 4 for transporting hydrogen, a cathode gas distribution plate 6 for transporting oxygen, and a cooling plate/humidifying plate 5. The anode gas distribution plate 4 and cathode gas distribution plate 6 may be combined to be a bi-polar plate 7.
Coating catalyst layer on the electrolyte membrane directly makes the catalyst slurry and the electrolyte membrane contact better, thereby decreasing reaction resistance and increasing activity. It is easier to control the thickness and quantity of the catalyst layer, thereby decreasing the quantity used and cost of the catalyst slurry.
However, the polymeric material of the electrolyte membrane available in the market, such as Nafion produced by DuPont or the products produced by Dow, Asahi Chemical, or Asahi Glass, has feature of good water adsorption. It is found that the polymeric material of the electrolyte membrane will absorb the solvent of the catalyst slurry when coating the slurry on the electrolyte membrane, thereby causing the electrolyte membrane expanding and deforming. With this character, it becomes a problem to coat the catalyst slurry on both sides of the electrolyte membrane evenly for making MEA.
For overcoming the problem of the electrolyte membrane deformation discussed above, a plurality of new manufacturing processes are developed. One of the new processes is coating the catalyst slurry on a transfer paper, then transferring the catalyst layer to the electrolyte membrane by hot pressing. FIG. 6 shows a conventional manufacturing process for manufacturing a fuel cell unit. The method comprises the following steps: preparing an electrolyte membrane 100, changing the property of the electrolyte membrane 101, coating catalyst layer on one side of the electrolyte membrane 102, pre-drying 103, coating catalyst layer on the other side of the electrolyte membrane 104, drying 105, changing the property of the electrolyte membrane again 106, finishing MEA 107, interposing the MEA between two gas diffusion electrodes and hot pressing 108, and finished the basic fuel cell unit 109.
In the prior art manufacturing method mentioned above, the property of the electrolyte membrane needs to be changed from H-form to Na-form by ion exchanging in step 101. Then, the Na-form membrane must be changed back to H-form in step 106 for purpose of proton transmission. In addition, the processes of this prior art must include coating a first catalyst layer on one side of the electrolyte membrane at first, pre-drying the electrolyte membrane coated with catalyst layer, coating a second catalyst layer on the other side of the electrolyte membrane, and drying the electrolyte membrane again. It is noted that the processes are so complex.
Accordingly, the object of the present invention is to provide a method for manufacturing membrane electrode assembly of a fuel cell for overcoming the problem of deforming when coating the catalyst slurry on the electrolyte membrane.
Another object of the present invention is to provide a simple method for manufacturing membrane electrode assembly of a fuel cell. The property of the electrolyte membrane doesn""t need to be changed when coating the catalyst slurry, and both sides of the membrane can be coated at the same time, making the manufacturing process much simpler.
Another object of the present invention is to provide a membrane electrolyte assembly with catalyst layers having fine structure for preventing from chapping or unevenness on the surface thereof. And the catalyst layer has nice contact with the electrolyte membrane thereby increasing reaction activity therebetween.
Another object of the present invention is to provide a method for manufacturing membrane electrode assembly of a fuel cell which can effectively control the thickness of the catalyst layer and also the quantity used of the catalyst slurry.
One more object of the present invention is to provide a method for manufacturing membrane electrode assembly of a fuel cell having better electric performance. Through the method of the present invention, the construction of the catalyst layer is improved, and also the contact between the catalyst layer and the electrolyte membrane is improved, thereby increasing the electric performance of the fuel cell.
To achieve the above objects, in accordance with the present invention, the electrolyte membrane is pre-expanded with solvent treatment and then at least one side of the electrolyte membrane is coated with catalyst slurry. The purpose of pre-expanding treatment is to prevent the electrolyte membrane from deforming when coating catalyst slurry on the membrane. Thereafter, the electrolyte membrane coated with catalyst layer is dried to evenly shrink the electrolyte membrane and the catalyst layer. By means of the processes of the present invention, the surface of the catalyst layer is even and not easily chapping, and the thickness and quantity of the catalyst layer is much easier to be controlled.
Preferably, before pre-expanding the electrolyte membrane, the organic matters on the surface of the membrane is removed by steps of soaking the electrolyte membrane in pure water, soaking the electrolyte membrane in H2O2 solution, while heating the electrolyte membrane at a temperature of 60xcx9c70xc2x0 C. for one hour, and cleaning the electrolyte membrane for removing organic matters from the electrolyte membrane.
Preferably, before pre-expanding the electrolyte membrane, the inorganic matter on the surface of the membrane is removed by steps of rinsing the electrolyte membrane with pure water 3xcx9c4 times, soaking the electrolyte membrane in H2SO4 solution, while heating the electrolyte membrane at a temperature of 70xcx9c80xc2x0 C. for one hour, rinsing the electrolyte membrane for removing inorganic matters from the electrolyte membrane, rinsing the electrolyte membrane with pure water 3xcx9c4 times again, and drying the electrolyte membrane at room temperature.
Preferably, the electrolyte membrane is soaked in alcohol such as Ethylene Glycol for at least 10 minutes.
Preferably, the catalyst layer is selected from the group consisting of Pt/C powder (20% Pt), Nafion solution (5 wt %), and Ethylene Glycol. The formula ratio of the Pt/C powder, Nafion solution, and Ethylene Glycol is Pt/C:Dry Nafion=3:1 by weight, and Ethylene Glycol:Nafion solution=1:1 by volume.
Preferably, the membrane electrolyte assembly fabricated by the present invention is further incorporated with a conducting plate and a graphite flow field plate onto the outer side of the membrane electrode assembly to form a fuel cell unit.